Programming Paradigms

Declarative language: What not how :

· Describe what should happen, not how to do it.

· The language should figure out how based upon the truths declared.

· No side effects: A rule is true And a function gives the exact same response regardless of the state of the system.

· No concept of mutable variables: In Excel, Cell A2 names a value, not a container to hold a value that changes.

· No sequence of commands. Express data flow, not command sequence. Example: Excel formula: =A1 + A2. (Use recursion, based on statements of facts, to flow.)

· Types:

· logic programming: Describes relationships in terms of inference rules.

o Example: Prolog

· functional programming: Describes relationships in terms of functions.

o Example: Haskell , Excel, and a subset of Racket.

Imperative: How not what:

· Exact rules for manipulation of the state of a computer system and its variables. Imperative does not have to consider the order or how it will be done. Think of commands to make something happen.

o Ex of pure imperative: SQL insert / update / delete

· Types:

o Procedural: Exactly how algorithms: Describe the exact steps to perform (using procedures or functions). Involves changing variables. First do this and next do that.

§ Example : C

o Object Oriented: Manipulates objects through predefined methods. Define objects in classes. Changes state of variables. Send messages between objects.

§ Example : C++, Java

Racket strengths:

· Adding language syntax: powerful macros allow programmer to add syntax constructs to extend the language

· continuations: allow you to capture the future value of a computation and store or pass it.

· dynamic typing: let the program figure out what type of variable is held. Saves program time.

· Manages its own memory.

· function on the fly (lambda): Easily create a function on the fly and pass it around. (Java allows, but not easily)

· function closure: a function created on the fly (lambda) can refer to variables in the scope of its creation. (Java limits the variables an on the fly function can reach.)

Great explanation lecture: http://www.infoq.com/presentations/Taming-Effect-Simon-Peyton-Jones

from MSDN: http://msdn.microsoft.com/en-us/library/bb669144.aspx

Characteristic	Imperative approach	Functional approach
Programmer focus	How to perform tasks (algorithms) and how to track changes in state.	What information is desired and what transformations are required.
State changes	Important.	Non-existent.
Order of execution	Important.	Low importance.
Primary flow control	Loops, conditionals, and function (method) calls.	Function calls, including recursion.
Primary manipulation unit	Instances of structures or classes.	Functions as first-class objects and data collections.

Sources:

http://foldoc.org/declarative+language

http://foldoc.org/imperative+languages

http://foldoc.org/procedural+language

http://people.cs.aau.dk/~normark/prog3-03/html/notes/paradigms_themes-paradigm-overview-section.html